1. Field of the Invention:
This invention is directed to a novel cement composition for the preparation of a novel aqueous cement slurry particularly useful in cementing casing in the borehole of a well. The novel cement composition comprises (1) an API Class "A" through "H" cement, (2) at least one polysaccharide having specially defined solubility characteristics, and (3) at least one salt.
2. Description of the Art:
After a borehole of an oil or gas well has been drilled, casing is run into the well and is cemented in place by filling the annulus between the borehole wall and the outside of the casing with a cement slurry, which is then permitted to set. The resulting cement provides a sheath surrounding the casing that prevents, or inhibits, communication between the various formations penetrated by the well. In addition to isolating oil, gas and water-producing zones, cement also aids in (1) bonding and supporting the casing, (2) protecting the casing from corrosion, (3) preventing blowouts by quickly forming a seal, (4) protecting the casing from shock loads in drilling deeper, and (5) sealing off zones of lost circulation. The usual method of cementing a well is to pump a cement slurry downwardly through the casing, outwardly through the lower end of the casing with a shoe and/or float valve and then upwardly into the annulus surrounding the casing. The upward displacement of the cement slurry through the annulus can continue until some of the cement slurry returns to the well surface, but in any event will continue past the formations to be isolated.
If the primary cementing of the casing, as described above, does not effectively isolate the formations, it may become necessary to perforate the casing at intervals along its length and then squeeze a cement slurry under high pressure through the perforations and into the defined annulus to plug any channels that may have formed in the cement sheath. Squeezing is an expensive operation that requires bringing perforating and cementing service companies back to the well and is therefore to be avoided, if possible.
It is critical in preparing cement compositions useful in cementing casing in the borehole of a well that they be characterized by a viscosity designed for optimum mixing at varying ambient temperatures, even at, or near, freezing temperatures, flow properties sufficient to facilitate and maintain lower laminar and/or plug flow and adequate gel strength to provide thixotropic properties to the slurry when pumping ceases.
Cement slurries, using conventional polysaccharide additives, thin or become less viscous with increasing temperatures, result in turbulent flow at high displacement rates, lose their suspension properties or capabilities as they become thinner or less viscous; and have a tendency to commingle with drilling fluids. If mixed with high-density additives, separation may occur at high temperatures, and such slurries are difficult to mix at low temperatures.
Slurries of decreased viscosities may channel through drilling fluids. Turbulent flow may also erode the wall of the borehole. If suspension properties are lost, water channels can be created in the slurries that allow gas, oil or water to migrate up or down (depending on the differential pressure and direction of pressure). Commingling of cement and drilling fluid results in contamination of the cement and will result in a poor cement bond and lower compressive strengths. When a slurry is mixed under freezing conditions, the slurry density can be lower than desired and will result in lower compressive strengths and insufficient hydrostatic pressures to contain formation pressures.